A gypsum board is a sheet-like body having a structure in which an upper face, a lower face, and left and right side faces of a gypsum slab are covered by lining paper sheets, and is widely used as building materials such as ceiling materials, wall materials, and flooring materials.
As a method for producing a gypsum board, the following production method for example is known. First of all, a laminated material in which a gypsum slurry is continuously injected into a gap between a pair of lining paper sheets are formed while continuously supplying the pair of lining paper sheets. In forming the laminated material, the lower lining paper sheet is hold upward along a marked line formed in the edge portions of both sides thereof. Thereby, the upper face of the gypsum slurry is covered by the upper lining paper sheet, and the lower side face and left and right side faces of the gypsum slurry are covered by the lower lining paper sheet. The laminated material formed in this way is allowed to pass between a pair of upper and lower forming plates, thereby obtaining a molded body having a thickness depending on a space between plates. A gypsum board is obtained as a final product by drying the molded body.
However, in such a production method, there has been a problem that when, for example, the gypsum slurry is contaminated by foreign bodies (such as a lump of gypsum), the foreign bodies are caught between a pair of upper and lower forming plates and the lining paper sheet is cut by the foreign bodies. The problem is brought about because the foreign bodies larger than the space between a pair of upper and lower forming plates are caught between the pair of upper and lower forming plates when the gypsum slurry containing the foreign bodies enters there, and therefore the progress of the lining paper sheet is inhibited at the portion where the foreign bodies are caught while the progress of the lining paper sheet continues to progress at the other portions. When the production is continued with the situation where the foreign bodies are caught left unchanged, the lining paper sheet continuously supplied is continued to be cut, resulting in continuous production of defective gypsum board products. Moreover, when such a situation is continued, the lining paper sheet is finally torn completely, and therefore not only supply of the lining paper sheet but also production of the gypsum board cannot be continued. In such a case, complicated operation in which the foreign bodies are immediately removed, then the lining paper sheets are supplied again between the pair of upper and lower forming plates, and the gypsum slurry is injected into the gap of the pair of upper and lower lining paper sheets to restart the production has had to be conducted.
In order to solve the problems as described previously, the present applicant has proposed an apparatus for producing a gypsum board constituted so that a space between a pair of upper and lower forming plates is expanded when the cut of the lining paper sheet is detected (Patent Literature 1). The apparatus includes, for example, as shown in an apparatus 100 for producing a gypsum board as illustrated in FIG. 4, a roll 136, an upper side forming plate 124, a lower side forming plate 108, an electrode (lower side external electrode 138) arranged on the downstream side of the lower side forming plate 108 apart from the lower side forming plate 108, an electric current detector 120, and an air cylinder 122. And a circuit 140 is constituted by electrically connecting the lower side external electrode 138 and the lower side forming plate 108, and the electric current detector 120 and a power source D are electrically connected to the circuit 140.
The apparatus 100 for producing a gypsum board makes it possible to detect the cut of the lining paper sheet by making use of the fact that the lining paper sheet is an insulator and the gypsum slurry is a good conductor of electricity. That is to say, in the apparatus 100 for producing a gypsum board, when the lower lining paper sheet 116 that is an insulator is cut, the gypsum slurry that is a good conductor makes contact with the lower side forming plate 108 and the lower side external electrode 138 and an electric current flows in the circuit 140, and therefore the cut of the lower lining paper sheet 116 can be detected by the electric current detector 120. Next, the air cylinder 122 lifts the upper side forming plate 124 depending on a signal from the electric current detector 120, thereby expanding the space between the upper side forming plate 124 and the lower side forming plate 108. Thereby, foreign bodies caught between the upper side forming plate 124 and the lower side forming plate 108 pass between the pair of forming plates. Thereafter, when the position of the upper side forming plate 124 is restored to the original position, production can be restarted. According to the production apparatus, the gypsum board can be produced continuously without stopping production and continuous production of a large amount of detective products can be avoided. Moreover, in addition to the detection of the cut of the lining paper sheet attributable to contamination of foreign bodies, the existence of defects can be detected because the gypsum slurry leaks out also in the case where defects (holes) are originally open in the lining paper sheet which makes an electric current to flow in the circuit 140.